It'd be a pain in the ass to make it so you couldn't reasonably access the customs folder.
It'd be a real pain in the ass to prevent you people from circumventing.
All in all, go ahead and access everyone's textures. The game needs to do that to display them, anyways. If you really do not to be able to access the textures, find a filesystem encryptor and encrypt the entire customs folder with an impossible to memorize password. Alternatively, delete system32.dll.